Link Aggregation is widely used to aggregate multiple links between a pair of nodes in order to be able to transmit user data on each of the links participating a Link Aggregation Group (LAG) (see, e.g., IEEE 802.1AX). Aggregating multiple network connections in this fashion can increase throughput beyond what a single connection can sustain, and/or can be used to provide redundancy in case of a failure of one of the links. The “Distributed Resilient Network Interconnect” (DRNI) project (see IEEE 802.1AXbq/D0.4) specifies extensions to Link Aggregation in order to be able to use link aggregation on a network interface involving two networks 1001, 1002 even between more than two nodes K, L, M, O, e.g. between four nodes as illustrated in FIG. 1.
The IEEE 802.1AX link aggregation standard does not force the assignment of a conversation to a single physical link. Therefore, the frames of a conversation may be transmitted on two different physical links in the two directions (i.e. the conversation would be considered “non-congruent” on the LAG). In case of a network interface, a conversation could be referred to as a “service,” since typically a service is handed-off through the interface. Using LAG for the interface links, it may happen that the service frames (e.g. IP packets) are handed off on one physical link in one direction but handed off on another physical link in the opposite direction, as illustrated in FIG. 2.
Congruent service hand-off, i.e. using the same physical link for frame transmission both in the forward and in the backward directions is highly desirable on a network interface. The service handed-off on an interface may be a Virtual Local Area Network (VLAN). Thus the Service identifier may be a VLAN Identifier (VID), such as a Service VID (i.e. “S-VID”) (typically identifying services on Network to Network Interfaces (NNIs)) or a Customer VID (i.e. “C-VID”) (typically identifying services on User to Network Interfaces (UNIs)). A VID-to-port mapping has been proposed by N. Finn in the “Issues to be discussed” portion of P802.1AXbq D0.3, Std. contrib. 2011 (http://www.ieee802.org/1/files/public/docs2011/bq-nfinn-DRNI-issues-0711-v1.pdf). This proposal provides a tool to select a physical link for a VID on one side of the nodes connected by the aggregated links. However, there is no means to enforce or even check whether the VID to physical link assignment is the same in both directions, i.e. that the service hand-off is congruent.